Open access peer-reviewed chapter
Abstract
Heatwaves have a growing impact on humans, ecosystems, and agriculture across Europe, while soil moisture and land cover represent key mitigation mechanisms endangered by the ongoing climatic change. Handling the situation as it evolves, with strong constraints on natural resources, is expected to become a major challenge, while health, ecosystems, and production systems will be under increased pressure. This chapter aims to present a state of research regarding the interaction between land cover and local climate in the context of global warming. The work is based on previous research and reviews completed for the present chapter. This opens further research perspectives assessing the soil-air-water interactions and climate, which is critical considering territorial planning. Heatwaves increased in frequency and intensity at the turn of the century. Global warming mechanisms affecting local warming and heat repartition can be somewhat influenced. From this perspective, permanent land cover, also endangered by climate change, plays a crucial role in mitigating climate at the local level. A detailed assessment of the change occurring in the Mediterranean region will be conducive to feeding further thoughts regarding upcoming challenges across Europe.
Keywords
- heatwave
- climate change
- global warming
- soil moisture
- heat tolerance
- land cover
- forest
- resilience
- mitigation
- temperature thresholds
- high temperatures
- evapotranspiration
1. Introduction
Notably, 2023 has been the hottest year since meteorological data were recorded, and 2024 [1] is about to beat the record. Since 2020, 3 years have been the hottest on record [2]. Heatwaves over Europe are becoming increasingly frequent and gaining intensity.
The Green House effect is a mechanism that allows Earth to keep its surface temperature within a range compatible with the development of life. The warming happens mainly because the atmosphere absorbs infrared wavelengths reemitted by the ground. There is nothing new in the fact that some gas in the atmosphere composition plays a prominent role in this regard. The composition of the atmosphere has itself changed over Earth’s geological times. Fluctuations, including cold and hot episodes, have occurred over Earth’s history. Past fluctuations can be assessed through geological analysis and, for the quaternary era, through ice probes analysing the gas composition and isotope proportions. The worst event occurred about 252 million years ago at the end of the Permian era [3, 4], leading to Earth’s worst life mass extinction. The massive eruption of an area as big as Alaska, now located in Siberia, led to the massive release of greenhouse gas (GHG) and a brutal temperature change. Considering less brutal fluctuations which prevailed throughout Earth’s history, the main difference between ongoing global warming and past events is related to the amplitude and speed of change.
Different factors intervene in Earth’s surface temperature fluctuation. The Sun emits more energy now than it did one billion years ago, and it also has cycles of activity that affect Earth [5]. The nature of the soil also modifies the radiative balance on Earth; oceans have a cooling effect through evaporation, as well as plants through evapotranspiration, while ice contributes to a direct reflection to Space. Besides, air, water, and soil interactions intervene in the repartition of energy balance at the globe’s surface. Oceanic currents play an important role in temperature repartition. An insight into meteorological data over the past provides a useful perspective regarding the ongoing challenge. Even though the thermodynamic regulation of the Earth’s atmosphere is a dynamic equilibrium that may evolve to some extent under global warming, both phenomena require clear distinction.
The World Meteorological Organisation defines heatwaves as ‘a period where local excess heat accumulates over a sequence of unusually hot days and nights’ [6]. Thus, thresholds regarding the maximum temperature during the day and minimal temperature at night are considered. In most cases, a heatwave will be defined as an abnormal deviation from average temperatures observed over a thirty-year reference period. Considering plants’ physiological limits like wheat, events involving a few days in a row with day temperatures over 30°C and/or tropical nights with temperatures not dropping below 20°C can be considered a definition. The definition can be much less restrictive, like the criteria established in 1947 by the French Meteorological Organisation [7], considering that temperature should be 1 day above 25.3°C and greater than or equal to 23.4°C for at least 3 days. Those episodes are not new, but an increase in the frequency or intensity of heatwave events is expected in the background of global warming. The role of land cover in climate attenuation at the local level and the local interaction with global warming is being acknowledged [8], as is the need for additional research to feed territorial management decisions. While new thresholds are bypassed, a global perspective on the impact on human health, forests, energy, agriculture, and retroactions between land cover and climate requires more attention.
The chapter is divided into five main parts (two to six). The methodology applied to the review is described at first. The following sections present a historical perspective illustrating the change in Ref. temperatures accompanying the heatwave concept, a description of the mechanisms triggering heatwaves, distinguishing earth’s global energetical balance from heat fluxes repartition mechanisms, the observed impacts and dangers faced by the vegetation land cover, and the projections from models (Figure 1).
Figure 1.
Heatwaves influencing factors and impact—© Julian Schlubach, 2024.
The conclusion aims to highlight key issues and define research to be conducted at the local level to clarify further the understanding of the challenges faced and what can be done.
2. Methodology
The present chapter continues previous work on the impact of global warming on field crops, considering plants’ physiological limits [9] and regarding the role of ecosystem services in adaptation to global warming [10]. It is opening perspectives on complementary research regarding territorial planning and mitigation measures. The aim is to provide a preliminary assessment of the tendency regarding heatwaves, their impact, and the extent to which the ecosystem services we may rely on from a climate mitigation perspective might be endangered. The proposed approach implies cross-cutting perspectives involving different research units and institutions. The present chapter also intends to establish a preliminary basis for discussions addressing the global warming challenge at the territorial level.
The core elements addressed under sections 4 to 6 are based on academic peer-reviewed research partly collected in the framework of previous research from the author [9, 10] and partly on additional references collected through a screening of research publication platforms. The climate and impact parts are based on peer-reviewed publications retrieved through research platforms like Science Direct, Elsevier, Springer, ResearchGate, or Nature. Publications accessible free of charge have been privileged, and some others have been purchased. The climate tendency described in Section 3 is mainly based on grey literature; Google and Wikipedia have been used for this purpose, and each reference has been cross-checked. Section 3 thus illustrates how the perception of temperatures and heatwaves has evolved while changes occur. The overall references used are not exhaustive but intend to illustrate important points that require attention, according to the state of the art. The analysed climate data related to meteorological references, including projections, in different European countries have been extracted from the World Bank climate open portal [11]. Meaningful available data to illustrate observed trends have been selected and extracted, even though averages are not optimal for analysis. The retrieved average temperatures have been put into perspective, and explanations regarding the interpretation are provided. The aim is to illustrate the main trends and challenges.
3. Historical perspective
Following the last glaciation period, the Holocene warming period reached a maximum estimated to have occurred 6500 years ago [12]. Different models show a global temperature decrease of 0.5°C until the industrial era. Thus, over time, Earth has slowly evolved towards a cooler climate. However, this trend stopped in the nineteenth century, with the industrial era. Although meteorological data do not allow much insight into recorded temperature beyond 1850, historical descriptions provide a perspective on the occurrence of extreme events. Some historically documented heatwaves were recorded across Europe, such as in 1540 [13], with extreme drought and hot temperatures over the year, and in 1808 [14] and 1858 [15] in the United Kingdom. Dendrology, assessing tree rings, provides a complementary insight into local climatic events, which have resulted in hydric stress in the past [16].
Over the twentieth century, at least nine heatwaves were reported in Europe, particularly in 1911, 1947, 1952, 1976, 1983, 1987, 1990, 1994, and 1997. In 1911, a heatwave across France and the United Kingdom (UK) beat a record of 36°C at the beginning of August for the UK [17]. In 1947, between the 27th of July and the 5th of August, maximal temperatures exceeded 40°C and a record of 37.6°C in Paris [18]. In 1952, Romania experienced a long hot day episode in August [19]; temperatures peaked at 40°C on the 17th, while abnormally high temperatures were also recorded at the beginning of October. The 1976 heatwave over North-Western Europe [20] lasted two consecutive weeks; day temperatures in the UK did not drop below 32°C, peaking at 35.9°C on July 3rd. In 1983 [21], a heatwave affected North-Western Europe over a few weeks in July, with temperatures reaching 38°C. In 1987, a one-week-long hot event affected an area extending from Italy in the South to the United Kingdom and Germany in the North. Registered day temperatures reached 40°C and barely dropped below 30°C at night [22]. Over a few days at the beginning of August 1990, temperatures peaked at 37°C in the UK [23] and reached 40°C in France [24]. A heatwave affected especially Poland between July and August 1994 [25], with temperatures reaching 39.5°C. In 1997, the United Kingdom faced one of the hottest summers on record, and the period from May 1997 to September 1998 was recorded as the hottest period worldwide on record [26].
Recorded heatwaves became more frequent at the beginning of the twenty-first century. At least eleven occurrences were recorded for Europe over twenty-three years: 2003, 2006, 2007, 2010, 2011, 2013, 2015, 2017, 2018, 2022, and 2023. The frequency is more than four times higher than in the past century. Besides, the intensity of the recorded events is unprecedented (Figure 2).
Figure 2.
Number of heatwaves reported in media between 2011 and 2023 per 38-year period—© Julian Schlubach, 2024.
In the United Kingdom, the record of 36.7°C set in 1911 was only beaten in 1990, when temperatures reached 37°C; hotter records have succeeded each other since 2000, reaching a record of 40.3°C on 19 July 2022 [27]. The same acceleration in the frequency and intensity of hot days is observed in France. The study of average temperatures from thirty meteorological stations since 1947 shows a steady increase in temperature records, with a considerable acceleration since the nineties [28]. Heatwaves in 2003 [29] and 2006 [30] affected most of Western Europe, breaking new temperature records with 47°C in the South of Portugal in 2003 and 40°C in Paris in 2006. The 2007 heatwave affected mainly the South of Europe, with heat extremes reaching 45°C in Bulgaria [31]. In June 2010 [32], a heatwave from the Sahara hit the East of Europe. In July 2015 [33], the West, including England, was affected, with temperatures reaching 37°C. In June 2017 [34], a heatwave affected Greece and the Balkans. In the summer of 2018 [35], Northern and Central Europe faced extreme heat and drought, and in July 2022 [36], most of Western Europe faced a heatwave. The United Kingdom experienced abnormal temperatures in October 2011 [37], July 2013 [38], and June 2017 [39]. In May 2022, a heatwave mainly affected Spain and France, among other European countries. The reached temperatures were the highest recorded in France since meteorological records exist, and a height of 41°C was recorded in Sevilla in Spain [36]. Summer temperatures in 2022 ranged from 2 to 4 degrees above the average registered over the preceding thirty-year reference period. In July 2023 [40], a heatwave mostly affected the Mediterranean region. In 2023, Temperatures were above average for 11 months of the year.
While heatwaves are not a new phenomenon in Europe, hot days lasting a few days in a row are becoming more frequent, with increasingly high recorded temperatures. Although it could be argued that this may enter a longer-term cycle, the occurrence of those events has to be placed in the context of the global climatic trend [26].
4. Mechanisms influencing heatwaves occurrence and intensity
Different mechanisms of energy exchanges between the atmosphere, oceans, and soils influence seasonal heat repartition on Earth. Air mass movement can induce rapid changes or increase the intensity of cold events or heatwaves. Global warming increases the overall energy intake at the Earth’s surface, modifying the local albedo and energy balance and possibly modifying some pre-existing dynamic equilibriums. At the ground level, the land surface radiative balance and soil moisture influence the climate. Thus, global warming increases the probability of heatwaves, while energy exchange dynamic mechanisms can intensify such events.
4.1 Global warming
At equilibrium, the earth reemits to space the same quantity of energy it receives. Until the nineteenth century, the climate was cooling [11], which means that the quantity of energy reemitted was slightly more important than the energy received. With the industrial era, the trend has been progressively reversed. Between 2000 and 2015, NASA reported an average net influx of 0.7 Watts per square metre (Figure 3) [41].
Figure 3.
Earth energy balance equilibrium (data from NASA [40])—© Julian Schlubach, 2024.
The difference in climate between Upper Normandy on the Atlantic North Coast of France and Provence on the Mediterranean Coast in the South of France is mainly due to the distribution of precipitation and the amplitude of temperature variations during the year. The average yearly precipitation over the 1991–2020 period was 823 mm for the Basse-Normandie Region and 849 mm in the Provence-Alpes-Côte d’Azur (PACA) region, with an average minimal temperature of 7.71°C in Basse-Normandie and 7.57°C in PACA, while average high temperatures are respectively of 15.19 and of 15.14°C [11]. However, these values also reflect that for average data, a minimal difference in measurement can represent a significant difference in seasonal and daily climatic variations for a given territory. Thus, the evolution of average annual temperatures for France, which oscillated around 13–14°C during the first half of the twentieth century to reach 16–17°C during the 2010–2020 decade, represents a development with observed impacts fuelling concerns.
In 2022, the average yearly mean temperature was 3.84°C for Sweden, 9.7°C for Poland, 10.11°C for the United Kingdom, 10.83°C for Germany [11], 13.16°C for France, 14.38°C for Italy, 14.88°C for Greece, and 15.3°C for Spain. The average value does not reflect the day and night and seasonal temperature variations. It is thus an indicator to handle with caution when considering vulnerability to climate change. In higher latitudes, heatwaves and droughts in summer are increasingly endangering ecosystems, even though temperatures are dropping to much lower levels than in Southern countries over the winter. Nevertheless, in the short term, the increase in average temperature does not have the same significance and impact in countries facing already higher temperatures. Besides, models present a slightly stronger summer temperature increase in Southern countries. However, considering the projected considerable increase in average temperature, tolerance thresholds in temperatures of ecosystems are likely to be increasingly frequently bypassed in all parts of Europe over the summer period. Continental areas face a higher challenge in this regard. Heatwaves are also likely to happen more often in spring and fall, with intra-seasonal, high-temperature variations, which could destabilise field crops.
4.2 Heat repartition: Intensifying and regulation mechanisms
Beyond Global Warming, three main factors related to energy repartition are presented hereafter: the jet stream under the polar vortex influence, the soil moisture, and the Atlantic Meridional Overturning Circulation (AMOC). In the present case, the cloud albedo variation is not addressed as a main factor of variation, as heatwaves are also correlated with strong anticyclones. The role of the AMOC and the way it might evolve is a mechanism regarding energy repartition on the earth’s surface to be considered in a longer-term perspective.
4.3 The polar vortex and the double jets
The polar jet stream is located at 50–60 degrees northern latitude. The subtropical jet stream is located around 30 degrees North at the level of North Africa. The jet streams are located, at about ten kilometres altitude, at the limit of the troposphere and of the Stratosphere. Over the polar circle, the polar vortex circles up in the stratosphere, influencing the jet stream below. Those high-altitude streams influence air masses on the ground, triggering North-South, as well as low-high-altitude air masses exchanges, which can result in rapid changes and extreme meteorological events on the ground. The polar vortex is occasionally split into two vortexes circling in the form of an eight, disrupting the jet stream below, South and Northward (Figure 4).
Figure 4.
Polar vortex and disruption in air masses circulation—© Julian Schlubach, 2024.
Abnormal hot events have been linked to the polar vortex disruption with a double jet stream [42]. A strong correlation has been established between those conditions and a stable, strong anticyclone over the North Atlantic, involving heat events over the United Kingdom, favouring hot and dry weather over Western Europe. The record of observations of the polar vortex disruption and double jet phenomenon covers an insufficient period to conclude an increase in the frequency of occurrence beyond possible natural cycles. However, models show that while over the Arctic Ocean, heat is absorbed by melting ice and water warming, the surrounding continental area’s rapid warming contributes to the shift of the equator to pole gradient towards the North Arctic Circle [42]. This ongoing change could favour double jet events. Warmer land areas in the Arctic could also favour the persistence of heatwave events. Double jet persistence correlates 30% with the variability in European heatwaves [43]. Thus, it is an important factor, but not the sole one.
4.3.1 Soil moisture
Soil moisture linked to soil temperature feedback influence on the local climate also plays an important role [43]. Global warming adds to the intensity (height of temperatures) and the cumulative intensity (cumulative excess temperature over a consecutive period of high-temperature event) (Figure 5).
Figure 5.
Evaporation and evapotranspiration cooling effect at the local level—© Julian Schlubach, 2024.
Early Summer soil moisture strongly correlates with heatwaves observed over Western Europe from 1980 to 2011 [43]. This is also the case around the Mediterranean Basin [44]. Water evaporation and evapotranspiration absorb energy, resulting in a cooling effect. While the soil surface dries up quicker, vegetation can mobilise water volumes stored in lower earth layers. The maximum volume of water stored depends on the nature of the soil. The effective volume of stored water is measured by an index of the ratio of the Volume of Water in a volume of soil [43]. The soil can release water until a wilting point beyond which the earth is too dry. The available water volume for evapotranspiration is the difference between the volume of stored water and the wilting point. If the early summer period is dry, the potential for evapotranspiration over summer is reduced. In the Mediterranean region, soil moisture limits evapotranspiration in the summer, while radiation limitation is the main limiting factor in the Arctic tundra [45]. Along the coats, the land benefits from the cooling effect of the evaporation over water masses. The climate rapidly changes when entering inland, and soil moisture becomes a determining factor. Forest cover plays a crucial role in this regard, allowing plants to mobilise bigger volumes of water from the soil [46]. The change occurs over small distances. In the Normandie region of France, three climatic zones are distinguished from the coastline while moving inland. The ‘oceanic climate benefits from cooler temperatures and more rain, while the ‘altered oceanic climate’ and the ‘degraded oceanic climate’ more inland present drier and hotter features [47]. In Central and Eastern Europe, the scope for climate regulation under global warming conditions is even more limited as soil moisture potential fades. A permanent land cover is of strategic importance for those areas.
4.3.2 Atlantic meridional overturning circulation (AMOC)
The European climate is influenced by the Atlantic Meridional Overturning Circulation (AMOC) [48], which transfers hot water from the Gulf of Mexico, warming the European coasts and atmospheric circulation, which turns according to the earth’s rotation. The AMOC is linked to the Coriolis forces resulting from earth rotation and may thus not disappear. However, changes in convection between deep ocean and superficial water may influence surface temperatures. A less strong current could also affect the heat transfer from the Gulf of Mexico towards the European coasts. The accelerated melting of the polar ice cap is modifying the water salinity, which may affect the deep-water flux along the North American coast. A current slowdown would affect heat repartition on both sides of the Atlantic and ocean-atmosphere dynamics. However, this may only marginally affect the tendency towards hotter summer and heatwaves occurrence. From this perspective, it may be noted that the cold water flowing down from the Arctic along the Canadian coast does not stop the occurrence of heatwaves, droughts, and subsequent devastating wildfires over the Canadian North.
5. Impact of heatwaves
Different parameters need to be taken into account when considering the impact of the exposure to high temperatures. The impact of heatwaves on ecosystems depends on the temperature intensity and prevailing drought conditions. Humidity, when associated with high temperatures and, in some cases, pollution, has a deadly effect on living beings. Heatwaves over the past two decades have resulted in a considerable increase in deaths, especially among elderly persons. Besides, changes in temperature and humidity are increasingly destabilising ecosystems. While heatwaves are succeeding, their impact can be observed, even though looking into the past will not properly inform either about upcoming changes.
5.1 Impact on human health and death toll
The 1911 heatwave has not been exceptional compared to the 1990–2020 reference periods. However, summer temperatures have been far above the recorded temperatures between 1870 (the first meteorological record in France) and 1945 [49]. No precise death toll linked to high temperatures has been recorded, while in Italy, excess death has been partly linked to cholera and possibly other diseases, which hot temperatures may have also favoured. Between 20 and 31 July 1987, a heatwave resulted in more than 700 excess deaths in Athens and its vicinity [50]. The 2003 heatwave was a turning point, with heatwaves across Europe resulting in the excess death of about 30,000 persons, with the highest toll in France and Mediterranean countries [51]. The summer 2022 heatwave resulted in more than 61.000 excess deaths across Europe [52]. Supplementary deaths were mainly registered in Spain, Italy, the Balkans, and Greece. The temperature deviation of 1 to 2 degrees in Germany and Scandinavia, with lower reference temperatures, resulted in less harsh absolute temperatures and a much lower resulting caseload.
5.2 Energy and watershed
Higher temperatures result in higher evaporation and evapotranspiration and in warmer water bodies and rivers. Consequently, Rain patterns are disrupted, resulting in drought and flood events. The water from the ice cap melt increases the amount of water available in rivers flowing down from high mountains. However, the snow cover’s progressive reduction, with strong interannual variation, jeopardises river flow over the summer.
Heated-up water in rivers increases the risk of algae blossoming and eutrophication. This evolving situation with reduced and warmer water availability raises concerns regarding thermal and nuclear plants requiring cooling. During the 2003 heatwave, some nuclear plants had to be put on hold. In France, an exemption to use water for cooling processes was granted to six nuclear and some thermic plants, even though the rejected water temperature bypassed environmental limits [51]. As people turn to air conditioning and production faces increasing constraints, the soaring electricity demand poses a challenge that will become more acute in a warming climate.
5.3 Impact on trees and forestry
While a historical perspective is lacking, there is no consensus regarding the degree of forest vulnerability [53]. However, this does not mean that the trend is not assessed as worrying. Different parameters can be assessed to measure the plants’ stress levels. The Vapour Pressure Deficit (VPD) measures the difference between the air’s water pressure and the plant’s vapour pressure, triggering evapotranspiration. The relative humidity the air can withhold rises with temperature. By approximation, the VPD is often considered to be the ratio of the air’s relative humidity to the theoretical maximum humidity at air saturation for a given temperature. The drier the air, the stronger the aspiration pressure through the plant’s leaves. Even a slight temperature increase creates a multiplicative elevation of VPD [54]. Besides, the quantity of water the plant can transport depends on the soil moisture, the size of the trunk, and the leaf area. The plant can regulate the flow by opening or closing the stoma in its leaf cells to different degrees, but under extreme conditions, this remains insufficient to regulate the flow in proportion to water intake and water loss.
The soil moisture at the root level provides information regarding water availability. The soil structure and its water content, jointly with the stand basal area and the root depth, define the volume of water available for a plant. Relative Extractable Water (REW) refers to the stock of water available in the soil, potentially available according to the soil structure. Thereafter, the Climatic Water Demand (CWD), expressed in pressure (kilo Pascal), quantifies the annual evaporative demand that exceeds available water. The Tree Water Deficit (TWD) is calculated at the plant’s level to measure the tree’s water transportation capacity [55]. It is based on the difference between the past highest stem radial record and the stem radial reading at a given time. Discrepancies between the TWD and the tree’s growth reduce the tree’s capacity to face a heatwave. The assessment of those factors, combined with meteorological data, satellite remote sensing imagery, and field data, allows us to fine-tune our understanding of local and global dynamics [56]. A longer-term perspective is required regarding land cover and tree die-off phenomena, as there is no direct link between a heatwave or drought event and the observed tree deaths. Evidence of delayed response following heatwaves has been documented since 2003 [57].
Trees’ adaptative mechanisms to stress and drought [58] are not at no cost to the plants. Those adaptative mechanisms include evapotranspiration regulation through stoma opening reduction, leaf area reduction, and physiological metabolism slowdown. Stoma closure under heat increases the risk of ‘hydraulic failure’ and ‘carbon starvation,’ which affect the plant’s metabolism and, thereafter, its capacity to cope with external aggressions. In the same way, a reduction in leaf area can substantially reduce the trees’ capacity to store carbohydrates. Thus, the plant’s capacity to recover after the shock is reduced [59], which ultimately can result in higher tree mortality. Increased CO2 availability in the atmosphere ceases to be an advantage for plant growth under high temperatures and drought conditions [60]. Photosynthesis is strongly reduced or stopped while the plant closes its stoma under heat stress. Besides, respiration, mainly at night, has a non-linear response to high temperatures [61]. This means that the physiological mechanisms linked to respiration deteriorate considerably or stop functioning beyond a temperature threshold. This, in turn, affects the plants’ capacity to recover and run other physiological mechanisms during the day, including photosynthesis.
Heatwaves increase water demand, resulting in drought when the Climatic Water Demand (CWD) exceeds water availability. The trees’ adaptative mechanisms, even to mild drought, weaken the plant’s resilience capacity. The plant’s capacity to cope when successive drought events occur, or other processes like pests or storms, is thus reduced step by step. Ultimately, this can lead to a die-off process. Multi-layers forests allow creating conditions for higher humidity retention, moderating temperatures under the canopy. The vegetation density can be an advantage in that regard. Higher die-off has been recorded in less dense stems per surface area [62]. Besides, a lower leaf area to sapwood (external wood rings allowing water and nutrient transportation) ratio reduces the risk of overcoming the Climatic Water Demand of the plant. This favours younger plants in comparison to old trees with broad leaf areas. As an adaptation strategy, some trees multiply young shoots. Exposure to high temperatures results in slower forest growth [63] and tree die-offs. Larger trees are at higher risk of dying [53].
Quick changes, like alternate precipitation extremes and heatwaves from 1 year to another or within a year, increase trees’ vulnerability [64]. Besides, there is an upper limit to the severity and frequency of shocks a tree can withstand, increasing mortality events over time [65]. A humid spring can result in increased leaf development, allowing higher evapotranspiration, but difficult to sustain during heatwaves in the summer period, as stems’ radial growth is reduced. This results in leaf loss and reduced plants’ resilience capacity [57]. Crown die-offs are most prevalent around rocky outcrops and in soils with poor water retention capacity. The probability of tree death diminishes with soil depth and stand basal area but increases with the trunk diameter at human breast height. In Central Europe, this process has led to an extensive ongoing dieback process [64]. However, the magnitude of the change cannot yet be evaluated. Predicting the future vegetation change and feedback to global climate is challenging. Diebacks have been reported on Norway spruce caused by different factors, possibly drought-induced on weakened trees [66]. European Beech (
Finally, the combination of high temperatures and dryer vegetation considerably increases the risk of wildfires. During the 2003 heatwave, more than 25,000 fires were recorded across Portugal, Spain, Italy, France, Austria, Finland, Denmark, and Ireland. In total, about 647,069 hectares were destroyed by fires across Europe. Between the reduction in evapotranspiration linked to high temperatures and wildfires, forest carbon sequestration is endangered. However, the primary role of forests in local climate mitigation remains crucial while considering those challenges.
5.4 Impact on agriculture
The 2003 heatwave has resulted in an estimated drop of 30% in primary productivity [60]. The heatwave began in early June, resulting in early crop development and ripening. However, further in July, the high temperatures, with high evapotranspiration, resulted in a water deficit affecting grain development [68]. Beyond the water availability for rain-fed field crops, like wheat, high temperatures over long periods, including at night, can also affect the plant’s physiological limits, affecting yields. The optimum temperature for wheat growth at the heading, anthesis, and grain filling duration is 16 ± 2.3, 23 ± 1.75, and 26 ± 1.53°C, respectively [69]. A high temperature above 40°C inhibits photosynthesis, reducing vegetative growth to the ‘zero level.’ Tropical night, where temperature does not drop below 20°C, also affects the plant’s physiological mechanisms. Over all of Europe, the main sectors hit by the extreme climate conditions were the green fodder supply, the arable sector, potatoes, the livestock sector, and forestry [69]. The fodder deficit varied from 30% (Germany, Austria, and Spain) to 40% (Italy) and 60% in France. The fall in cereal production in the European Union reached more than 23 million tonnes as compared to 2002. Herders have been suffering from rising fodder prices. The 2018 heatwave affected an area 50% larger than in 2003. Pastures and arable land have been especially affected [70]. In 2018, gains in Maize yields were estimated to be 10% in Romania and Hungary, but losses of 10% occurred across Germany and Belgium, resulting in an average loss of 6% for Europe. Even though forest mortality and growth decline are occurring with delay, according to the latest trend, Germany’s forestry estimated loss amounted to 105 million cubic metres in 2018.
Since the end of the Second World War, yields have considerably increased with the joint optimisation of varieties and inputs. A more detailed assessment of the quantitative gain compared to the quality of the considered crop varieties would be required to draw precise conclusions. Nevertheless, the production gain has been real. Thus, the gain in productivity over the past period might blur the assessment of counterproductive effects linked to drought and heatwaves. Besides, multiple factors can also affect the crop development. Nevertheless, a study comparing the occurrence of Extreme Weather Disasters (EWD), droughts or heatwaves, and crop production anomalies over the 1964–1990 and the 1991–2015 period concluded that in the second period, production losses in Europe saw an average cumulative increase of 3% per year [71]. Heatwaves have been assessed as being twice as damaging for cereals than for other crops.
Higher temperatures during the crop’s vegetative development and even more at critical development stages, like heading, anthesis, and grain filling for cereals, increase the risk of stepping over the plant’s physiological limit. While global warming progresses, agriculture will likely face an increasing challenge in maintaining yields. Where, in the past, the main limiting factor was considered to be drought, high temperatures will require much more attention in the forthcoming years. Even if stable in frequency, the disruptions of the North Pole vortex cold events are likely to affect crops that will bud earlier under warming conditions. Abnormal warm conditions followed by cold events could become a more common phenomenon disrupting field crops and overall ecosystems in spring. Nevertheless, shifts in the cultivation calendar may improve the resilience of annual crops. Cooling through spraying water and drip irrigation for high-added-value crops can also mitigate the effect of heatwaves. However, drip irrigation in open fields will face limits when the temperature rises beyond the plant’s physiological limit, while spray irrigation will be confronted with reduced water availability in summer. Genetic modification can offer some scope to maximise plants’ resistance to different stress factors. However, this potential does not mean the absence of limits. Plants fixing carbon from the atmosphere on a four-carbon chain (C4) have better resilience to high temperatures than plants fixing carbon on a three-carbon chain (C3) [72]. This is because the C4 plants create a stronger partial pressure gradient at the leaf level, which allows for the absorption of more CO2 from the atmosphere with a smaller opening of the stoma. Gas exchanges between the leaf cells and the air can thus take place with less water loss from evapotranspiration. However, there is no scope for transforming C3 into C4 plants. Besides, C4 crops like maize [73] and sorghum also require considerable amounts of water and have upper limits to heat tolerance, about 35°C, a threshold above which vegetative growth is stopped. Beyond the possible improvements, by specialising excessively plants on limited genetic criteria, the genetic intra-specific variety among individuals is strongly reduced, affecting their capacity to face a changing environment. Increased resistance to drought or slightly higher temperatures might be done at the cost of lower resistance to other aggressions, which may occur. The advantage of annual crops is that the sowing season can be changed annually, while perennial crops or trees with an even longer lifespan require more anticipation. Global warming will increasingly affect European countries from the South to the North. According to prevailing local conditions, ecosystems, and water availability, permanent adaptation to changing conditions will thus be required.
6. GHG emission trend and heatwave occurrence
6.1 GHG emissions and global warming scenarios
Since the first world conference on climate change was held in Rio in 1992 and, more recently, since the Paris Declaration in 2015, GHG emissions have increased year after year. In 2023, with 37.4 Giga tonnes (Gt), emissions increased further by 1.1% compared to 2022.
Far from falling rapidly—as is required to meet the global climate goals set out in the Paris Agreement—CO2 emissions reached a new record high of 37.4 Gt in 2023 [74]. Atmospheric CO2 concentration reached 419.3 parts per million (ppm), 51% above pre-industrial level (around 278 ppm in 1750). In practice, the countries’ pledges presented in Nationally Determined Contributions (NDC) are declarations of intentions that are not likely to be met and, in most cases, only consider a reduction of emissions in proportion to a growth scenario [75]. Therefore, the agreed effort is to slow the increase rather than decrease emissions. Representative Concentration Pathways (RCP) are elaborated according to different assumptions. Shared Socio-economic Pathways (SSP) have been defined for each RCP, defining possible societal choices influencing the RCP scenario, including adaptation plans. SSP5/RCP 8.5 has been developed as a no-action reference scenario. It considers a continuous increase in GHG emissions without change, the ‘business as usual’ scenario. In 2024, it seems to be the scenario most closely reflecting the reality of the pathway as it happens. Comparisons between projections in energy production modelling scenarios and RCP 8.5 present a 90–98% convergence for CO2 and CH4 emissions [76]. RCP 8.5 has been the most consistent model in coherence with observed historical trends in CO2 emissions (within 1% for the 2005–2020 period); it also remains a plausible path until 2100 under current policy perspectives [77].
6.2 Heatwave occurrence in Europe according to models
For France, on average, in a scenario with +4°C at the end of the century, the average annual number of heatwave days would be eight to ten times higher in 2100 in comparison to the 1976–2005 reference period [77]. On the French Mediterranean coast, the increase would be eleven to twelve times. Under RCP 8.5, by 2071–2100, heatwaves could start to occur in early May until mid-October [78]. The heatwave episodes are expected to become longer, up to 2 months in the summer period, with temperatures up to 6°C above the maximum observed over the 1976–2005 reference period.
According to the SSP5/RCP.8.5 model, in France, the average heating by 2080–2099 compared to the 1995–2014 period would be 3–4°C on average over the winter period and above 6°C between July and September [11]. For Germany, the projections show an increase of 4.5 to 6°C between June and October, with a rise above 5°C from July to September. For Germany and Poland, the average maximal temperature for the 1995–2014 reference period was 12°C. For Sweden, the increase in average maximal summer temperature is similar to the one foreseen for Germany and Poland, but starting from substantially lower reference temperatures, with an average maximal temperature of 6°C for the 1995–2014 reference period [11].
Temperature variation does not have the same meaning everywhere, but it will result in different challenges depending on the geographic area considered. In Sweden, the increase in summer temperature will affect the snow cover and glaciers, while changing rain patterns might additionally increase the risk of droughts. High seasonal temperatures and droughts could thus endanger forests. In Southern Europe, high seasonal temperatures will more rapidly become a threat to human health and production systems while jeopardising ecosystems.
7. Conclusion
We lack temporal distance to evaluate the possible disruption and changes in energy fluxes at the earth’s surface. However, the acceleration of heatwave events in frequency and intensity since the 1990s and even more into the twenty-first century underlines the effect of global warming. Meteorological phenomena can worsen or attenuate the trend, but they do not deeply change the dynamic humanity and all living beings face. Heatwaves beyond a temperature threshold, which varies among crops and varieties, stop plant growth, jeopardise yields at critical development stages, and endanger ecosystems. Such events will also increasingly affect territories in Southern Europe and progressively in Northern countries. Besides, recurrent heatwaves with increasing intensity endanger trees and forests all over Europe. This could put the vegetation at risk and reduce the attenuation effect. Mediterranean and Central European continental areas will be exposed first. Over the twenty-first century, sustaining vegetation permanent land cover will be a major challenge, while its reduction will affect the capacity to mitigate local climate.
However, land cover remains a key element among attenuation mechanisms that can be influenced. Forest cover influences the local climate, reduces temperature, and favours more regular rain patterns. Vegetation’s role may thus not be reduced to carbon sequestration. Sufficient interspecific and intra-specific biodiversity is important to maximise the resilience of those ecosystems. However, urban sprawl and, in some cases, extension of annual crops or the change of land destination with solar panel fields instead of natural areas affect the local albedo and evapotranspiration. Adaptation mechanisms can also help increase ecosystems and crop resilience locally. The sowing period can be adjusted, some improvements in varieties can be made, and irrigation techniques can be applied, among other mechanisms. However, those solutions also have weaknesses, affecting production systems’ resilience and implying choices in water allocation. In the short term, annual crops have more scope for adaptation, while perennial crops or trees with an even longer lifespan might require more anticipation. Northern European countries have a bit more time, even though the 2003 and 2018 heatwaves showed they need to be prepared for the upcoming change. In any case, adaptation measures need to be defined on a case-by-case basis. However, reducing greenhouse gases should be a priority, requiring decoupling economic growth scenarios from energy consumption. The increasing demand for air cooling devices in summer will be challenging in this regard. Besides, at the local level, territorial planning requires strategic insight to attenuate, as far as possible, the effects of global warming.
Further research is required to assess the local impact of global warming on living systems and possible attenuation measures at a territorial level. This research should associate meteorological data, a network of forest tree observations measuring soil moisture, VPD, and die-out count, and broader satellite imagery soil moisture data. Assessing land cover dynamics, LAI, and crop yields could add relevant complementary insight. The work to be conducted in the South of France and possibly in neighbouring Mediterranean countries could provide useful conclusions to allow more Northern European countries to anticipate forthcoming changes.
Acknowledgments
The present publication is based on the work of the author, bibliographic references, and experience drawn from different parts of the world.
Conflict of interest
The author declares no conflict of interest.
Appendices and nomenclature
Atlantic Meridional Overturning Circulation carbon dioxide plants absorbing carbon from the atmosphere on a three-carbon chain plants absorbing carbon from the atmosphere on a four-carbon chain climatic water demand extreme weather disasters greenhouse gas intergovernmental panel on climate change leaf area index—measuring leaf surface available for evapotranspiration land use, land use change, and forestry nationally determined contributions Provence-Alpes-Côte d’Azur (Region—France) representative concentration pathway relative extractable water shared socio-economic Pathways tree water deficit United Kingdom vapour pressure deficit
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